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Re: Truck stopping distances?



Regarding Joseph B.'s comment:

With all due respect, do you really think this long analyse, given the
list of approximations made, says anything more than "more initial KE,
requires more braking, requires more distance so that the brakes don't
get too hot"?

Well, it says a little more, but, admittedly, not a whole lot more.
My analysis was given as an attempt to provide some further support
for Joseph's initial observation (shot 2) via a scaling argument. I
didn't expect that Joseph wouldn't welcome the support.
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Regarding John D's. comment:

David Bowman wrote:
...
dissipation
...
The geometry of the vehicle shape approximately requires that the
braking surface area scales as the second power of the linear size L
of the vehicle.

Nice try. Alas, "approximately requires" is not the same as
"requires".

Have you ever seen the brakes on a DC-10?

No.

Most people
haven't, but they're quite something. Each brake is
a giant Dagwood sandwich: Stator, rotor, stator, rotor,
stator et cetera. You wind up with the dissipation taking
place over lot more surface area than Dr. B's scaling
argument would suggest.

BTW the actuator on the DC-10 is pretty impressive, too.
Hydraulic rams mash the sandwich. There are nine rams:
three run by the #1 hydraulic system, three run by the
#2 hydraulic system, and three run by the #3 hydraulic
system. But we digress.

Very interesting. However, I had thought we were mostly comparing
road vehicles trucks/cars that *do* have crudely similar brake
designs, except for overall scale. Aren't truck brakes mostly just
large size versions of normal drum brakes whose overall geometry is
similar to that of automotive brakes in that the drums have a
similar proportional relationship to the tire's size? Isn't the main
difference the possible use of compressed air as the hydraulic fluid
rather than brake fluid?

But if you leave out the mention of dissipation, the
scaling argument holds up better. My best guess is that
trucks are limited by the strength of materials, namely
the strength of the rubber in the tires.

I think John's point here may, indeed, be more important than the
problems associated with the braking surface temperature limits.

Thanks, John, for pointing this out.

See last week's
discussion of wide-track tires. Trucks don't have as
much tire area as they "should" in proportion to their
weight, relative to cars. They make up for it with
higher tire pressure. Lots higher.

And the belts are tougher and the rubber composition is harder and
stronger, as well, to be able to properly withstand that higher
pressure and higher vehicle weight on a proportionally small
footprint.

The sideways force on each molecule of rubber is
proportionately higher. At some point you start
erasing the rubber right off the tire.

This is certainly true. As I recall from talking with a tire design/
testing engineer many years ago back in Akron, I think their industry
jargon term for this erasing/erosion phenomenon is 'abrasion', and
the rubber composition exposed on the surface is said to 'abrade'
away from the tire tread. But I think I like the nicely vivid term
'erasing' better, though.

David Bowman
David_Bowman@georgetowncollege.edu